Hypoxic regions of solid tumors are the source of therapeutic-resistant fractions of tumor cells. The oxygen-regulated Hypoxia-lnducible Factor (HIF)-1alpha transcription factor is a master regulator of the hypoxic response, and is over-expressed in approximately 29% of primary breast tumors and approximately 79% of metastases. Moreover, HIF-1alpha has recently been demonstrated to down-regulate estrogen receptor (ER)alpha, suggesting that tumor hypoxia contributes to tamoxifen-resistance. Yet, the specific contribution of HIF-1alpha to breast cancer remains unclear. Based on these observations, I hypothesize that HIF-1alpha is a critical positive factor in breast tumor progression and metastasis and functions to promote tumor cell adaptation to the microenvironment. In addition, I hypothesize that HIF-1alpha-dependent genes are differentially regulated between normal breast tissue and tumors and that a unique subset of genes in the breast is co-regulated by hypoxia and estrogen. I have extensive experience using genetic mouse models to understand normal mammary gland development. Now, as a new Assistant Professor at the University of Tennessee Cancer Institute (UTCI), I seek to define the role of HIF-1alpha in breast cancer etiology. I will test my hypotheses through the following aims: 1) To determine the functional significance of HIF-1alpha conditional deletion in the mammary epithelium upon breast cancer progression and metastasis. I previously developed a mouse model to delete HIF-1alpha in the mammary epithelium using the Cre/loxP system. This model will be bred to transgenic mice that develop mammary tumors. The tumor latency, growth rate, histology and gene expression profile will be compared between wild type (WT) and HIF-1alpha null tumors. Stable cell lines from WT and null tumors will also be generated for further in vitro analysis. 2) To identify HIF-1alpha target genes in the normal gland vs. mammary tumors and to identify HIF-1alpha target genes co-regulated by estrogen. The differential expression profile of HIF-1alpha-dependent target genes between WT and KO normal mammary tissue and the mammary tumors derived in Aim 1 will be identified using Affymetrix microarrays. Next, microarray profiling will be used to determine the subset of HIF-1alpha target genes that is co-regulated by estrogen in the ERalpha+ breast cancer cell line MCF-7. The ultimate goal of these studies is to identify the mechanisms by which HIF-1alpha promotes breast tumorigenesis and metastasis in order to develop a translational research program immediately relevant to breast cancer patients.